Vitamin B12 - CyanocobalaminThe name vitamin B12 (or B12 for short) is used in two different ways. In a broader sense it refers to a group of Co-containing compounds known as cobalamins - cyanocobalamin (an artefact formed as a result of the use of cyanide in the purification procedures), hydroxocobalamin and the two coenzyme forms of B12, methylcobalamin (MeB12) and 5-deoxyadenosylcobalamin (adenosylcobalamin - AdoB12). In a more specific way, the term B12 is used to refer to only one of these forms, cyanocobalamin, which is the principal B12 form used for foods and in nutritional supplements.
Pseudo-B12 refers to B12-like substances which are found in certain organisms, such as Spirulina spp. (blue-green algae, cyanobacteria). However, these substances do not have B12 biological activity for humans.
B12 is the most chemically complex of all the vitamins. B12's structure is based on a corrin ring, which, although similar to the porphyrin ring found in heme, chlorophyll, and cytochrome, has two of the pyrrole rings directly bonded. The central metal ion is Co (cobalt). Four of the six coordinations are provided by the corrin ring nitrogens, and a fifth by a dimethylbenzimidazole group. The sixth coordination partner varies, being a cyano group (-CN), a hydroxyl group (-OH), a methyl group (-CH3) or a 5'-deoxyadenosyl group (here the C5' atom of the deoxyribose forms the covalent bond with Co), respectively, to yield the four B12 forms mentioned above. The covalent C-Co bond is the only carbon-metal bond known in biology.
B12 cannot be made by plants or by animals, as the only type of organisms that have the enzymes required for the synthesis of B12 are bacteria and archaea. The total synthesis of B12 was reported in 1973 by Robert Burns Woodward, and remains one of the classic feats of total synthesis.
Coenzyme B12's reactive C-Co bond participates in two types of enzyme-catalyzed reactions.
Rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcochol, or an amine.
Methyl (-CH3) group transfers between two molecules.
In humans there are only two coenzyme B12-dependent enzymes:
MUT which uses the AdoB12 form and reaction type 1 to catalyze a carbon skeleton rearrangement (the X group is -COSCoA). MUT's reaction converts MMl-CoA to Su-CoA, an important step in the extraction of energy from proteins and fats (for more see MUT's reaction mechanism)
MTR, a methyl transfer enzyme, which uses the MeB12 and reaction type 2 to catalyzes the conversion of the amino acid Hcy into Met (for more see MTR's reaction mechanism).
History as a treatment for anemia
B12 deficiency is the cause of several forms of anemia. The treatment for this disease was first devised by William Murphy who bled dogs to make them anemic and then fed them various substances to see what (if anything) would make them healthy again. He discovered that ingesting large amounts of liver seemed to cure the disease. George Minot and George Whipple then set about to chemically isolate the curative substance and ultimately were able to isolate vitamin B12 from the liver. For this, all three shared the 1934 Nobel Prize in Medicine.
The chemical structure of the molecule was determined by Dorothy Crowfoot Hodgkin and her team in 1956, based on crystallographic data.
B12 is mostly absorbed in the terminal ileum. The production of intrinsic factor in the stomach is vital to absorption of this vitamin. Megaloblastic anemia can result from inadequate intake of B12, inadequate production of intrinsic factor (pernicious anemia), disorders of the terminal ileum resulting in malabsorption, or by competition for available B12 (such as fish tapeworms or bacteria present in blind loop syndrome). Neurological signs of B12 deficiency, which can occur without accompanying hematologic abnormalities, include demyelination and irreversible nerve cell death. Symptoms include numbness or tingling of the extremities and an ataxic gait.
The American Psychiatric Association's American Journal of Psychiatry has published studies showing a relationship between depression levels and deficient B12 blood levels in elderly people in 2000 and 2002 .
Traditionally, treatment for B12 deficiency was through intramuscular injections of cyanocobalamin. However, it has recently been appreciated that deficiency can be treated with oral B12 supplements when given in sufficient doses. The usual daily intake in the Western diet is 5-7 mcg. However, when given in oral doses ranging from 1000-2000 mcg daily, B12 can be absorbed in a pathway that does not require an intact ileum or intrinsic factor. The Schilling test can determine whether symptoms of B12 deficiency are caused by lack of intrinsic factor, though this is being performed less often due to the lack of availability of reagent for the test.
Vitamin B12 is naturally found in foods including meat (especially liver and shellfish), eggs, and milk products. Fortified breakfast cereals are a particularly valuable source of vitamin B12 for vegetarians and vegans. Table 1 lists a variety of food sources of vitamin B12.
While lacto-ovo vegetarians usually get enough B12 through dairy products or eggs, it may be found lacking in those practicing vegan diets who do not use multivitamin supplements or eat B12 fortified foods, such as fortified cereals (including Cheerios), fortified soy-based products, and fortified energy bars. Claimed sources of B12 that have been shown through direct studies of vegans to be inadequate or unreliable include spirulina (an algae), nori (a seaweed), barley grass, and human gut bacteria. Several studies of vegans on raw food diets show that raw food offers no special protection against B12 deficiency either. The only known vegan sources of substantial B12, aside from multivitamin supplements and fortified foods, are the Chinese herb Dang Gui (Angelica sinensis), used for centuries for treating anemia, and certain brands of nutritional yeast.
Interestingly certain insects such as termites have been found to contain B12.
Cyanocobalamin is also found in many energy drinks, such as Steven Seagal's Lightning Bolt and Red Bull.
Other useful Vitamin information: Inositol | Vitamin B-9 | Vitamin B-2
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